1. Field of the Invention
The present disclosure relates to a liquid crystal display and, more particularly, to a fringe field switching (FFS) mode liquid crystal display in which resistance of a common electrode is reduced, and a manufacturing method thereof.
2. Background of the Invention
In general, a driving principle of a liquid crystal display (LCD) uses optical anisotropy and polarization of liquid crystal. Since liquid crystal is thin and long in structure, having molecule arrangement directionality, and alignment of molecular arrangement may be controlled by artificially applying an electrical field to liquid crystal.
Thus, when a direction of a molecule arrangement is adjusted, a molecule arrangement of liquid crystal is changed and light is refracted in the direction of the molecule arrangement of liquid crystal due to optical anisotropy, expressing image information.
Currently, an active matrix (AM)-liquid crystal display (LCD) in which thin film transistors (TFTs) and pixel electrodes connected to the TFTs are arranged in matrix form has come to prominence due to excellent resolution and video implementation capability.
The LCD includes a color filter substrate (i.e., an upper substrate) on which a common electrode is formed, an array substrate (i.e., a lower substrate) on which pixel electrodes are formed, and liquid crystal filling a gap between the upper substrate and the lower substrate. In the LCD, the liquid crystal is driven by an electrical field applied up and down to the common electrode and the pixel electrodes, exhibiting excellent transmissivity, aspect ratio, and the like.
However, driving liquid crystal by an electric field applied up and down is disadvantageous in that viewing angle characteristics are not good.
Thus, in order to overcome the shortcomings, a fringe field switching (FFS)-mode liquid crystal driving method has been newly proposed. A method for driving liquid crystal based on the FFS mode provides excellent viewing angle characteristics.
An existing FFS-mode LCD device having the forgoing advantages will be described with reference to FIGS. 1 through 3.
FIG. 1 is a plan view of an array substrate of an FFS-mode LCD according to a related art.
FIG. 2 is a cross-sectional view taken along lines Ila-Ila and Ilb-Ilb of FIG. 1, illustrating an array substrate of the related art FFS-mode LCD.
Referring to FIGS. 1 and 2, an array substrate 10 for the related art FFS-mode LCD includes a plurality of gate lines 13 extending in one direction and parallel with each other on a substrate 11; a plurality of data lines 23 intersecting the gate lines 13 to define pixel regions in intersections therebetween; and TFTs each formed at the intersections between the gate lines 13 and the data lines 23 and each including a gate electrode 13a, an active layer 19, a source electrode 23a, and a drain electrode 23b. 
A large transparent pixel electrode 15 is disposed to be spaced apart from the gate lines 13 and the data lines 23, and a plurality of bar-shaped common electrodes 29a formed of a transparent ITO material are disposed above the pixel electrode 15 with a gate insulating layer 17 and a passivation layer 25 interposed therebetween and common electrodes 29b overlapped with the gate line 13 and the data line 23.
The pixel electrode 15 is electrically connected to the drain electrode 23b through a drain electrode connection pattern 29c in contact with the drain electrode 23b through a drain contact hole 27a and a pixel electrode contact hole 27b formed in the passivation layer 25 and the gate insulating layer 17. A ohmic contact layer 21 is disposed at between the active layer 19 and the source/drain electrodes.
FIG. 3 is a cross-sectional view illustrating a common electrode having a single layer structure overlapped on the data line and the gate line in the array substrate for the related art FFS-mode LCD, in which (a) is a cross-sectional view illustrating a common electrode overlapped on the data line and (b) is a cross-sectional view illustrating a common electrode overlapped on the gate line.
Referring to (a) and (b) of FIG. 3, the common electrodes 29b overlap the gate line 13 and the data line 23 with the passivation layer 25 interposed therebetween, respectively, are configured as a single layer, and formed of a transparent conductive material.
According to the configuration, when a data signal is supplied to the pixel electrode 15 through the TFT T, a fringe field is formed between the common electrodes 29a and 29b to which a common voltage has been supplied and the pixel electrode 15, and thus, liquid crystal molecules arranged in a horizontal direction between the TFT substrate 11 and the color filter substrate (not shown) are rotated due to dielectric anisotropy, and as transmissivity of light passing through the pixel region is varied according to a degree of rotation of the liquid crystal molecules, a gray scale is implemented.
As described above, recent product models employing the related art FFS-mode LCD have been increased in size to satisfy product diversity together with high transmissivity in order to meet the requirements of high resolution.
However, the common electrode applied to existing medium-sized LCDs is formed of an ITO material having high specific resistance and applies a common voltage signal to the interior of a display region therethrough. Here, since the common electrode is formed of a transparent conductive material such as ITO, resistance of the common electrode is continuously increased, but due to an advantage of improvement of transmissivity, it is difficult to design an extra common line.
Also, due to the increase in resistance of the common electrode, the existing LCD has problems in which a quality risk such as a residual image, a color shift, and the like, is increased.
Also, in case of an existing large LCD, a common signal needs to be applied to the interior of the display region through an additional common line, but the disposition of the additional common line reduces transmissivity.
Thus, in case of a large LCD, since transmissivity needs to be continuously enhanced, the additional common line causing a degradation of transmissivity needs to be omitted.